Such methods and corresponding predistorters are per se known from prior art and they are used to shape an input signal before said input signal is fed into a power amplifier. The power amplifier itself usually has a nonlinear characteristic which is undesirable since it causes the input signal to be distorted, i.e. a gain imparted on said input signal by said power amplifier is not constant but depends on an amplitude of said input signal.
To avoid this unwanted distortion, the input signal is predistorted by a predistorter prior to feeding it to said power amplifier. The predistorter distorts or shapes, respectively, the characteristic of the input signal in a manner opposite to the distortion caused by said power amplifier, and in a well-tuned system an overall distortion of said input signal may thus be avoided. Said predistortion is usually achieved by multiplying said input signal by a correction signal or by adding such a correction signal which depends on said input signal.
Contemporary methods use so-called polynomial predistorters wherein the correction signal is based on a polynomial. Since each function value of a polynomial depends on all coefficients of said polynomial, it is not possible to tune the correction signal or certain intervals of it, respectively, via changing one or few coefficients without changing the complete correction signal. As a further disadvantage, said polynomial predistorter only works well for a limited range of input amplitudes and is thus not flexible with respect to a varying mean power of said input signal. Furthermore, the polynomial predistorter is not flexible enough with respect to special required shapes of a characteristic of the non-linear power amplifier.
Another solution known from prior art employs a lookup table to determine function values of said correction signal. Consequently, it is possible to configure a predistorter wherein each function value depends on one parameter that can be read from said lookup table. This solution also lacks flexibility, because especially in adaptive systems, where a dynamic modification of said parameters is required, the number of parameters that must be updated is too high.
The present invention relates to a method of influencing an input signal, in particular an input signal of a power amplifier of a wireless communication system, wherein said input signal has a range of input amplitudes, wherein a correction signal depending on said input signal is generated, and wherein said input signal is influenced depending on said correction signal.
The present invention also relates to a predistorter for influencing an input signal, in particular an input signal of a power amplifier of a wireless communication system, wherein said input signal has a range of input amplitudes, wherein a correction signal depending on said input signal is generated, and wherein said input signal is influenced depending on said correction signal.
Therefore, it is an object of the present invention to provide an improved method of influencing an input signal and a corresponding predistorter which provide increased flexibility.